1. Field of the Invention
The present invention relates to catalysts for the synthesis of alkylene carbonates by reacting alkylene oxide and carbon dioxide.
2. Description of the Background Art
Alkylene carbonates are used in polycarbonate synthesis, as a solvent for polymer electrolyte, an intermediate in pharmaceutical process, an oxyalkylation agent in dyestuff synthesis, a protectant in processing plant and a solvent in textile production process.
Alkylene carbonate has been prepared by reacting carbon dioxide and alkylene oxide in the presence of a catalyst, represented in Scheme 1. 
wherein, R1 and R2 are each independently H, C1-C4 alkyl or phenyl group.
In the above reaction, however, there is a limitation that alkylene oxide either decomposes or polymerizes at higher reaction temperatures.
Many catalysts have been developed including inorganic salts, phosphonium halide and ammonium halides. For instance, Japanese Laid-Open Patent No S59-13776 introduced a method of using tetraalkyl halide such as tributyl methyl phosphonium iodide as a catalyst. Japanese Laid-Open Patent No H9-67365 introduced a method of using KI as a catalyst and Japanese Laid-Open Patent No. H9-235252 describes a method of using polystyrene copolymer containing quaternary phosphonium groups.
These patents claim that the product yield is 50-95% when the reaction is performed at 100-170xc2x0 C. for 1-5 hours. However, in order to achieve a high yield, longer reaction time and higher reaction temperature are required. Also the water content in the raw materials, carbon dioxide and alkylene oxide has to be reduced to a few hundred ppms.
Japanese Laid-Open Patent No. H7-206846 introduced a method of using an ion change resin substituted with the catalysts such as CsOH, RbOH and ammonium halides. In U.S. Pat. No. 4,233,221, a method of using DOWEX and Amberlite ion exchange resin was reported with a low yield of 30-80% at 80-100xc2x0 C.
Besides the above-mentioned materials, a phthalocyanine complex containing Co, Cr, Fe, Mn, Ni, Ti, V, or Zr has been used as catalysts Also in Japanese Laid-Open Patent No. H7-206847, a catalyst system using a heteropolyacid whose hydrogen ion is substituted by Rubidium or Cesium ion was introduced These two cases, however, require expensive catalysts with low yield of 30-90% at relatively high reaction temperature of 120-180xc2x0 C.
As mentioned above, the catalysts disclosed in the above arts have one or more problems in terms of activity, reaction condition, cost, water sensitivity, etc.
Therefore the object of the present invention is to provide catalysts for the synthesis of alkylene carbonates from alkylene oxide and carbon dioxide with a high yield and selectivity in a short reaction time under a mild reaction condition
The present invention provide a catalyst of the formula (1) for the synthesis of alkylene carbonate by reacting alkylene oxide and carbon dioxide
LmMXn xe2x80x83xe2x80x83(1) 
wherein L is selected from a group of pyridines;
M is a metal atom selected from Zn, Fe, Mn, Pb and In;
X is a halogen ayom selected from Cl, Br and I;
m is 1 or 2; and
n is 2 or 3.
In particular, the catalyst of the present invention is used to synthesize alkylene carbonate of the formula (2) 
wherein R1 and R2 are each independently H, C1-C4 alkyl or phenyl.
The present invention also provide a method for synthesizing alkylene carbonate from alkylene oxide and carbon dioxide by using the catalyst of the formula (1)